The condom slipped once inside her and she took plan B... four days later the condom slipped inside her again ...what are the chances of her getting pregnant?

You don't want to know the chances of her being pregnant. You want to know if she is pregnant. An old stats teacher told the joke: "All chances are 50-50. Either an even will happen, or it won't. " The chances of her being pregnant is a meaningless idea. Have her take an otc test, or take a trip to her doctor, or wait a few weeks.